1. Field of the Invention
The present invention relates to so called positive electric safety switches, with automatic or manual control, for opening an electric circuit.
2. Description of the Prior Art
In well known devices of this kind, a push rod or lever controls and electric contact when it is actuated in certain privileged directions, with end of travel or angle clearance.
Such devices are difficult and even dangerous in use, for the degree of freedom of the feeler element is limited and if it is accidentally or involuntarily driven beyond its limits it may be damaged.
Devices are also known which use the breaking of rigid elements, rods or conducting plates, but these devices have the drawback that the component which is destroyed must be dismantled and replaced by an identical component after each operation.
To overcome these problems, French patent 2. 284 178 proposes an electric switch comprising an insulating case supporting two electric terminals connected to the circuit to be broken. A removable conducting rod, comprising an extension forming operating member, is intended to come to bear against the terminals so as to establish electric contact. The rod is normally held in a stable abutment position, pushed by a retaining piece urged by a spring and operable by means of a resetting lever, against the electric terminals and against a case portion forming a fixed guide. The rod may escape from the retaining piece, after reversible resilient travel, under the action of a force applied to its extension, and thus open the electric contact.
Such a device has the advantage of being able to break the circuit following action on the rod in any direction, the circuit being broken without destroying any member, the rod being able to be replaced by the user as often as desired. However, it has been discovered that this device has the disadvantage of momentarily breaking the circuit, either when it is subject to vibrations, or when the rod is actuated incompletely. In particular, this device finds an interesting application for providing safety on cable transporting pylons, the rod being intended to break the circuit when it is pushed by the cable at the time of a derailment. Frost present on the cable may produce on the rod thrusts and movements insufficient to cause it to be released from the retaining piece, but sufficient to move it momentarily away from an electric terminal and break the circuit. Furthermore, the pylons are subjected to considerable vibrations; the rod, held simply by the retaining piece, may oscillate in its housing against the thrust of the spring and, following vibrations, produce temporary breaks in the circuit. Furthermore, the vibrations produce premature wear of the contacts, causing rust to appear also at certain places.
The document Research Disclosure volume 173 page 90 describes a simplified switch for cameras, comprising two spring wires wound on two respective non conducting terminals. A first end of the two wires forms conductors for the electric connection to a circuit. The second end of one of the wires forms a transverse connecting rod bearing resiliently on the turns of the other wire so as to establish electric contact, this second end being able to be operated by a push rod. In this device, the resilient rod provides the main electric contact.
This document gives no information about the existence and the solution of the problems related to breaking speed and to the presence of temporary breaks during the resilient travel of a switch according to patent No. FR-A-2 284 178.
The present invention has more especially as object to avoid the drawbacks of this known device by proposing improvements for avoiding the temporary opening of the electric circuit due to incomplete actuation of the rod or to vibrations.
Another object of the invention is to provide permanent cleaning of the electric contacts, while avoiding wear and breakage thereof following vibrations.
According to another object of the invention, the improvements increase substantially the breaking speed of the switch, particularly in the case of rod movements which gave rise before to temporary opening of the electric circuit.